Charred bone: Physical and chemical changes during laboratory simulated heating under reducing conditions and its relevance for the study of fire use in archaeology

In order to gain insight into the timing and nature of hominin fire use, the effect of heat on the physical and chemical properties of the materials entering the archaeological record needs to be understood. The present study concerns the fire proxy heated bone. Two types of heating can be distinguished: combustion (or burning, with oxygen) and charring (without oxygen), for both of which the formation of char is the first step. We performed a series of controlled laboratory-based heating experiments, in reducing conditions (i.e. charring), covering a broad temperature range (20–900 °C), and applied a variety of different analytical techniques. Results indicate that charred bone shows a distinctly different thermal alteration trajectory than combusted bone, which has implications for the suitability of the different analytical techniques when identifying and determining past heating conditions (charring vs. combustion; temperature) of heated bone from archaeological contexts. Combined, the reference data and techniques presented in this study can be used as a robust toolkit for the characterisation of archaeological charred bone from various ages and contexts.

The carbon (δ13C) and oxygen (δ18O) isotope compositions of fossilized animal tissues have become important proxies of paleodiet and paleoenvironment, but such stable isotope studies have not yet been extensively applied to the fossil assemblages of Sundaland (the biogeographical region comprising most of the Indonesian Archipelago). Here, we use the isotope composition of tooth enamel to investigate the diet and habitat of bovids, cervids, and suids from several Holocene and Pleistocene sites on Java and Sumatra. Our carbon isotope results indicate that individual sites are strongly dominated by either C3-browsers or C4-grazers. Herbivores from the Padang Highlands (Sumatra) and Hoekgrot (Java) cave faunas were mainly C3-browsers, while herbivores from Homo erectus-bearing sites Trinil and Sangiran (Java) utilized an almost exclusive C4 diet. The suids from all sites show a wide range of δ13C values, corroborating their omnivorous diet. For the dataset as a whole, oxygen and carbon isotope values are positively correlated. This suggests that isotopic enrichment of rainwater and vegetation δ18O values coincides with an increase of C4-grasslands. We interpret this pattern to mainly reflect the environmental contrast between glacial (drier, more C4) and interglacial (wetter, more C3) conditions. Intermediate herbivore δ13C values indicating mixed C3/C4 feeding is relatively rare, which we believe to reflect the abruptness of the transition between glacial and interglacial precipitation regimes in Sundaland. For seven Homo erectus bone samples we were not able distinguish between diagenetic overprint and original isotope values, underlining the need to apply this isotopic approach to Homo erectus tooth enamel instead of bone. Importantly, our present results on herbivore and omnivore faunas provide the isotopic framework that will allow interpretation of such Homo erectus enamel isotope data.